The present invention relates to a system that directs a finely focused ion beam onto a substrate to be patterned and moves the position at which the beam impinges on the substrate in order to write a desired pattern on the substrate.
A system which is capable of focusing ions into a fine beam and deflecting the beam can be used for an ion implantation system having no mask. (A focused ion beam producing system used in another application is described in U.S. Pat. No. 3,889,115.) This ion implantation is achieved by causing an ion source to produce doping ions, focusing these ions into a beam, and directing the beam to impinge upon a substrate material in a desired pattern. The substrate material may consist of silicon, gallium arsenide, or another material. Since the position at which the beam falls on the substrate can be moved by deflecting the beam, any desired pattern can be drawn on the substrate by controlling the deflecting of the beam according to the desired pattern. As a result, the doping (impurity) ions are implanted into desired locations on the substrate.
When ions are being implanted into a crystalline semiconductor substrate material, if the ions find entry into an open space between rows of atoms channeling starts to occur. Then, the impurity ions penetrate deep into the open space (channel). Accordingly, it is required that the ions be inserted off the center of the channel, depending on the purpose. Unfortunately, however, such channels frequently run vertically to the surface of the material. Therefore, it has been the common practice to incline the surface of the substrate material at an angle of about 7.degree., for example, to the direction of the implantation of the beam, or the optical axis of the equiment used.
This prior art equipment is shown in FIG. 1, where a material 1 to be patterned is inclined at an angle to the optical axis 0 of the equipment. Also shown are an objective lens 2 and an electrostatic deflector 3. An ion beam (IB) is finely focused onto the target material 1 by the objective lens 2. The position at which the beam irradiates the target material is varied according to the deflecting voltage supplied to the deflector 3. It is assumed that the beam is brought into focus when the beam falls on the target material surface at a position lying on the optical axis 0. If the beam is then deflected so that the beam falls on the target material surface at another position not lying on the optical axis 0, then the beam is defocused, i.e., the diameter of the beam deteriorates. This defocusing arises from the deflection of the beam and also from the inclination of the material. That is, this undesirable phenomenon stems from the fact that the distance between the starting point of the beam deflection and the irradiated position on the material 1 varies widely according to the deflection of the beam. This requires that the equipment be adjusted to change the feeding voltage of the objective lens 2 in a wide range, whereby the ion beam irradiating the material 1 is maintained in focus, irrespective of the deflection angle. It is very cumbersome, however, to control the voltage of the objective lens in this way, because the deflection must be effected according to both the deflection angle and the inclination angle of the target material. Further, when the target material 1 is moved to the position indicated by the broken line in FIG. 1, the position of the focused beam on the optical axis is shifted. This makes it more difficult to achieve automatic adjustment of focus (dynamic focusing).
In view of the foregoing, the present inventor has already invented an ion beam lithograph system shown in FIG. 2, where a material 1 to be patterned is disposed substantially vertically to the optical axis 0 of the ion beam. A newly introduced deflector 4 deflects the beam at a constant angle so that the beam impinges on the material 1 at a substantially constant incidence angle to prevent the beam from penetrating deep into the target material.
Although the position at which the ion beam strikes the target material 1 is varied by deflecting the beam through the use of the deflector 3, the distance between the starting point of the deflection of the beam and the illuminated position varies only slightly according to the deflection angle of the beam defined by the deflector 3, because the flat surface of the material is disposed substantially vertically to the optical axis 0. Therefore, the focus is automatically adjusted only with respect to the deflection angle. Hence, the objective lens and other elements can be easily controlled.
My aforementioned invention provides an ion beam lithography system that is able to focus the ion beam quite easily. Usually, the energy spread of ions produced from a liquid metal ion source are about 10 eV under the condition of low emission current. Since the ion beam having such an energy speed is deflected, the ion beam is dispersed greatly at the target plane. The deflection chromatic aberrations caused by the deflection increase the diameter of the beam on the surface of the target material, making it impossible to draw a pattern with a beam diameter less than 0.1 .mu.m.